1. Field of the Invention
This invention relates to a technique for detecting a rear wheel lifting phenomenon in which a rear wheel lifts from a ground surface, and to a technique for controlling a brake based on the detection, in a two-wheeled vehicle which is equipped with an antilock brake control unit (later referred to as an ABS-unit).
2. Related Art
A rear wheel lifting of this type is liable to occur especially in two-wheeled vehicles. Among all types of two-wheeled vehicles, the rear wheel lifting more easily occurs, in those two-wheeled vehicles in which a ratio between a height of center of gravity and a distance from a front wheel to a rear wheel, is larger. For example, if a front wheel should be braked in a two-wheeled vehicle having such a large ratio while the vehicle is running on a road surface having a large slip friction coefficient (so called high xcexc), a rear wheel would be easily lifted in air. It is desired to properly control the braking with respect, for example, to a front wheel by detecting a rear wheel lifting in order to ensure running-stability of the vehicle.
In a case where a two-wheeled vehicle having a large ratio between a height of center of gravity and a distance from a front wheel to a rear wheel, is equipped with an ABS-unit, the rear wheel lifting possibly causes the braking control to be deteriorated. For example, when a rear wheel lifting occurs (in the lifting state, a ground load of the rear wheel becomes zero), the ABS-unit becomes difficult to receive feedback information from the road surface, the computation of an estimated vehicle control speed in the ABS-unit is jeopardized and the braking is deteriorated.
The idea itself for detecting a rear wheel lifting in a two-wheeled vehicle and a brake controlling is conducted in accordance with the detection, is known as disclosed, for example, in Japanese Patent Application Laid-Open Publication No. H05-24523 (corresponding to European Patent No. 524821 A2). This Publication teaches a technique in which the lowering of a rear wheel ground load is detected by monitoring changes in wheel speed at the rear wheel (namely, wheel acceleration and deceleration at the rear wheel) of a two-wheeled vehicle. It is difficult, however, to correctly detect the lowering of the ground load based on the wheel acceleration and deceleration at the rear wheel, if it is taken into account that the wheel acceleration and deceleration at the rear wheel becomes a positive/negative value in accordance with the wheel controlling cycle when the ABS-unit is being antilock controlled and that the state of a road surface changes. In order to prevent erroneous detection, it is necessary to take a longer timer setting time. This causes to lower the sensitivity in detection.
[Objects of the Invention]
It is, therefore, a problem to be solved or first object of the present invention to provide a technique capable of detecting lifting of a rear wheel in a two-wheeled vehicle correctly and rapidly.
Another object of the present invention is to provide a brake controlling technique capable of conducting a proper brake controlling operation by rectifying an estimated vehicle speed in antilock brake controlling operation based on detection of a rear wheel lifting.
[Means for Solving the Problems]
The inventor of the present invention has carried out running tests of two-wheeled vehicles under many different conditions and made extensive study on the results. Consequently, he has noticed that when a rear wheel lifting occurs, significant changes are taken place in which deceleration in estimated vehicle speed obtained by computation based on a so-called select high, is abruptly decelerated. In the present invention, attention is paid to changes in estimated vehicle deceleration and the rear wheel lifting phenomenon in a two-wheeled vehicle is detected by utilizing such changes. In comparison with the conventional detection technique, the detection technique according to the present invention is a technique for monitoring the degree of estimated vehicle deceleration representing a state of the vehicle, while the conventional one is a technique for monitoring wheel acceleration and deceleration representing a state of the wheel. When a two-wheeled vehicle running on a high xcexc road surface is fully braked, the estimated vehicle deceleration is originally at a level representing a high degree of deceleration and relatively stabilized. However, the degree of wheel acceleration and deceleration is changeable depending on motion of the wheel, and it changes when the antilock controlling operation is conducted, irrespective of the state of the road surface and the state of the vehicle. For this reason, from the view point of timer-setting short in order to avoid erroneous detection, the detection technique according to the present invention can be said to be superior to the conventional one.
In a technique for detecting lifting of a rear wheel according to the present invention, an estimated vehicle speed is generated by selecting the larger one of wheel speed of a front wheel and a rear wheel at the time of detecting lifting of the rear wheel from a ground surface, and attention is paid to an estimated vehicle deceleration which is computed based on the estimated vehicle speed and it is judged that lifting of the rear wheel has occurred when a degree of lowering of the estimated vehicle deceleration is a predetermined value or larger. At the time of an actual judgment, the degree of lowering of the estimated vehicle deceleration can be judged as a predetermined value or larger when one of the following items A and B is judged to be present;
A. A first condition is satisfied in which the estimated vehicle deceleration is lowered than a predetermined value and in addition, a second condition is satisfied in which the first condition is fulfilled for a predetermined time or longer.
B. A third condition is satisfied in which the estimated vehicle deceleration is lowered than a predetermined value within a predetermined time.
With respect to the item A, the first condition about changes in estimated vehicle deceleration is the condition which is based on the finding of the inventor of the present invention, and the second condition about continuation of time is a condition corresponding to the previously-mentioned timer setting time. The item B relates to a method for setting conditions without using a timer, in which an average value of changed amounts is set by a so-called time window and the average value is monitored. Since the item B is eased in conditions for continuation of time compared with the item A, lifting of the rear wheel can more easily be detected. Although both methods of the items A and B can effectively be applied to two-wheeled vehicles of the type in which resistance is small about an axle of a rear wheel as a driven wheel, the method of the item B is more preferable than the method of the item A with respect to two-wheeled vehicles of the type in which resistance is large about an axle of a rear wheel.
For example, in two-wheeled vehicles of the type in which resistance is relatively large about an axle of a driven wheel, it sometimes occurs that the second condition under the item A becomes unable to be satisfied because the vehicle speed is slightly reduced due to resistance of a driving system of a rear wheel which is lifted, when this occurs, in air. In this respect, since the degree of vehicle deceleration is monitored as an average value in the item B, the rear wheel lifting can more easily be detected.
On the other hand, in a technique for controlling a brake according to the present invention, rear wheel lifting is detected by detection technique utilizing changes in estimated vehicle deceleration, and a deceleration gradient of the estimated vehicle speed used for the ABS-unit is limited to be small based on the detection of lifting of the rear wheel. Moreover, preferably, by computation of the estimated vehicle speed based either on the front wheel speed or on a lowest wheel speed (so-called select low) at the time of detection of lifting of the rear wheel, the estimated vehicle speed is eventually made compliant with the wheel speed of the front wheel.